Social networks and digital cameras have led to substantial video and media content produced by individuals each day. Hence, video understanding and analysis continues to be one of the essential research subjects in computer vision. While deep learning has accomplished remarkable performance in many computer vision tasks, video understanding is still far from ideal.
Action understanding, notably, as a vital element of video analysis, facilitates the advancement of numerous real-world applications. For instance,
collaborative robots need to recognize how the human partner completes the job to cope with the variations in the task [1]}. Sport analysis systems must comprehend game actions to report commentaries of live activities [2]}. Autonomous driving cars demand an understanding of operations performed by the surrounding cars and pedestrians [3]}.
